1. Field of the Invention
The present invention relates generally to supplemental respiratory devices and, in particular, to those supplemental devices that deliver respirable gas intermittently, based upon user demand.
2. Background of the Invention
Individuals who have a chronic obstructive pulmonary disease, or other form of respiratory insufficiency, often require supplemental oxygen therapy. This therapy involves enriching the air they breathe with enough oxygen to compensate for their reduced respiratory capacity, and thus, to insure they achieve adequate blood-oxygen levels.
Supplemental respiratory systems generally utilize a nasal cannula, connected to a pressurized oxygen source, to deliver the oxygen to the user. Originally, these systems were designed to administer oxygen on a continuous basis, wherein gas was allowed to flow to the user without interruption. However, this delivery mode wasted gas. Oxygen allowed to flow during exhalation or between breaths would dissipate into the ambient air, and thus, would be of no therapeutic value to the user. Various respiratory devices have been developed that are directed towards the goal of reducing this waste, and thereby, reducing the cost of oxygen therapy.
The basic operating principle of these improved devices is to intermittently deliver the gas, on demand, i.e. to supply the gas only during the inspiratory phase of the user's respiratory cycle. Current art respiratory devices utilize fluidic means to sense the negative or positive pressure, relative to atmospheric pressure, that occurs in the user's airway during inspiration and expiration respectively. An example of a fluidic device is seen in U.S. Pat. No. 4,414,982 to Durkan. The Durkan device uses fluidic sensing and gas regulating means to control the flow of gas, from a pressurized gas source, to the user. The Durkan device, however, requires the use of a specialized split nasal cannula that pneumatically isolates its fluidic sensing apparatus from the fluidic shock associated with the onset of each gas delivery. Such split cannulae are more expensive and less available than standard nasal cannulae. A demand regulated respiratory device, as described in U.S. Pat. No. 4,461,293 to Chen, can be used with a standard nasal cannula, and in this regard, is easier and less costly to use than the Durkan device. Also, whereas the Durkan device uses fluidic means for both sensing the user's respiration, and regulating the delivery of oxygen, the Chen device incorporates electronic means to regulate gas delivery. In general, electronic components are smaller and lighter than comparable fluidic devices; thus, electronic components can allow the design of more lightweight and compact, and hence, more portable respiratory devices. Portability is an important consideration, as many individuals who require supplemental oxygen therapy need such respiratory assistance on a long-term basis. Therefore, a portable unit can enhance their mobility, and thus, their quality of life.
Various respiratory devices utilize a fixed delivery approach, whereby the gas is allowed to flow to the user for a set period of time, for each respiratory cycle. However, fixed time respiratory systems can deliver more oxygen than is needed by the user. If the set delivery period exceeds the duration of the user's inhalation, which can occur at the high breath rates common to individuals requiring respiratory supplementation, a portion of the delivered gas will arrive after the inspiratory phase has ended, thereby conferring no therapeutic value to the user.